Turbomolecular pump

ABSTRACT

A turbomolecular pump ( 10 ) comprises a stator ( 12, 18 ), a pump rotor ( 20 ) a motor ( 28 ) for driving the pump rotor and a control device ( 42 ). The control device ( 42 ) controls the motor output power such that the motor output power does not exceed a permissible maximum motor output power. On the stator side of the turbomolecular pump ( 10 ), temperature sensors ( 32–38 ) for measuring the stator temperature are arranged. The control device ( 42 ) comprises a maximum output power detecting device ( 50 ) that determines the permissible maximum motor output power in dependence on the measured stator temperature. Thus, the permissible maximum motor output power is not set to a constant value but always fixed in dependence on the current, measured stator temperature. Thereby, the capacity of the motor can be fully utilized as long as the measured stator temperature lies below a maximum value.

BACKGROUND OF THE INVENTION

The invention relates to a turbomolecular pump with a pump stator, afast rotating pump rotor and a motor for driving the pump rotor.

In a turbomolecular pump, a gas or gas particles are compressed byrotating blades of the pump rotor and the stationary blades of the pumpstator to a multiple of the supply pressure to generate a high-vacuum.The gas heating caused by the gas compression and gas friction is mainlydissipated again. via the pump rotor and the pump stator. While thecooling of-the pump stator can be effected by cooling channels carryinga cooling fluid, the active pump rotor cooling is problematic since nocooling fluid can be supplied to the rotating pump rotor. Underunfavorable operational conditions, the pump rotor may thereforeoverheat. In case of an overheating of the pump rotor beyond a maximallypermissible rotor temperature, there is the danger of destroying thepump rotor and, as a consequence, the pump stator. Therefore, theturbomolecular pump always has to be operated below the maximallypermissible rotor temperature.

A direct measurement of the rotor temperature is only possible at greatefforts because of the difficult signal transmission from the fastrotating pump rotor to the stator. Therefore, the turbomolecular pumpcomprises a control device restricting the motor output power to apredetermined constant maximum motor output power so that the pumpoutput power and the gas and rotor heating correlating therewith arerestricted to a constant maximum value as well.

The permissible maximum motor output power is detected by calculatingand/or experimentally by assuming the most unfavorable processconditions for the pump operation, such as a gas with a thermallyunfavorable behavior, a bad pump stator cooling, high ambienttemperatures etc. The permissible maximum motor output power is selectedso that the pump rotor cannot exceed the maximally permissible rotortemperature even under the most unfavorable process conditions. Byfixing a constant maximum motor output power, the motor output power isrestricted to the predetermined maximum output power even if the processconditions are more favorable than assumed for calculating the maximummotor output power. Thus, the motor output power is restricted to thepredetermined maximum motor output power even if the actual rotortemperature has not reached the maximally permissible rotor temperatureyet. Since the extreme process conditions underlying the detection ofthe maximally permissible maximum motor output power only represent arare exceptional case in practice, the output power of theturbomolecular pump is normally restricted to a value far below anactually thermally permissible value.

Therefore, it is an object of the invention to provide a device and amethod by means of which the output power of a turbomolecular pump isincreased.

SUMMARY OF THE INVENTION

According to the invention, a temperature sensor for measuring thestator temperature is arranged at the pump stator. Further, the controldevice comprises a maximum output power detecting device determining thepermissible maximum motor output power in dependence on the measuredstator temperature. This means that the permissible maximum motor outputpower is no constant invariable value but is determined in dependence onthe respective stator temperature. The rotor temperature stronglycorrelates with the temperature of the stator-side parts of the pump,with, for example, the temperature of the base flange, the pump housing,the motor housing, the bearing housing, the pump stator, the motor aswell as the actual motor and pump output power, respectively. Therefore,the stator temperature gives information about the rotor temperature sothat also the rotor temperature can be reliably restricted to a maximumvalue by measuring the stator temperature and restricting thepermissible maximum motor output power for the respective statortemperature. By measuring the stator temperature and the conclusionsthat can be drawn therefrom with respect to the rotor temperature, thepermissible maximum motor output power is adapted to the respectivethermal situation and therefore normally lies above a constantpermissible maximum motor output power determined for most unfavorablethermal conditions. The actual motor output power and thus the outputpower of the pump can thus be clearly increased under normal processconditions. At the same time, the pump rotor is protected more reliablyagainst overheating, i.e., exceeding the maximally permissible rotortemperature, since an indirect monitoring of the rotor temperature iseffected.

According to a preferred embodiment, the maximum output power detectingdevice comprises a rotor temperature detecting device detecting therotor temperature from the stator temperature measured by thetemperature sensor. Subsequently, the maximum output power detectingdevice determines the permissible maximum motor output power independence on the detected rotor temperature.

The rotor temperature detecting device detects the motor rotortemperature from one or more different stator temperatures substitutedinto a polynomial the constant coefficients of which have been detectedexperimentally before. Thus, the permissible maximum motor output powercan be finally detected fast and with little memory space as well. Ifnecessary, the restriction of the maximum motor output power may notintervene until a threshold temperature of the rotor is reached, andrestrict the permissible maximum motor output power while the maximummotor output power is not restricted as long as the calculated rotortemperature is below the threshold temperature. The permissible maximummotor output power may also be detected directly from a polynomialresolved according to the permissible maximum motor output power and inwhich the rotor threshold temperature and/or a rotor maximum temperatureis already included in the form of coefficients.

The maximum motor output power calculated on the basis of thecoefficients may even be additionally restricted by other parameters, ifnecessary.

Preferably, several temperature sensors are provided at different sitesof the stator, the maximum output power detection device determining thepermissible maximum motor output power in dependence on the measuredtemperatures of all temperature sensors. The temperature sensors can bearranged at the housing of the turbomolecular pump, at a pump statorelement, at a stator-side part of the motor, e.g., at the motor housingor at the motor winding, or in a cooling channel of the pump stator. Thetemperature sensors can also be arranged at other stator-side sites ofthe turbomolecular pump the temperature and temperature behavior ofwhich permit reliable conclusions with respect to the temperature of therotor. Thus, from a plurality of measured temperatures, a preciseconclusion with respect to the rotor temperature and thus thepermissible maximum motor output power is made possible. Therefore, therestriction of the motor output power is effected close to theobjectively permissible maximum motor output power. The detection of therotor temperature and the permissible maximum motor output power byseveral stator-side temperature sensors is so reliable and precise thatonly small safety margins have to be provided to avoid an overheating ofthe rotor. Thus, the motor can be driven with a maximum of thermallypermissible output power, i.e., the output power potential of the motorand the pump can always be approximately completely utilized.

According to a preferred embodiment, the maximum output power detectingdevice comprises a characteristics diagram memory in which thepermissible maximum motor output power for each stator temperature isstored in a characteristics diagram. In the characteristics diagram, acomplex non-linear characteristics line can be stored as well so that acomplicated detection of the permissible maximum motor output power bycalculating operations can be omitted.

According to another method for restricting the maximally permissiblemotor output power of a motor in a turbomolecular pump, which drives apump rotor borne in a pump stator, the following method steps areprovided: measuring the pump stator temperature, detecting a permissiblemaximum motor output power from the measured pump stator temperature,and restricting the motor output power to the detected permissiblemaximum motor output power.

Still further advantages of the present invention will become apparentto those of ordinary skill in the art upon reading and understanding thefollowing detailed description of the preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may take form in various components and arrangements ofcomponents, and in various steps and arrangements of steps. The drawingsare only for purposes of illustrating a preferred embodiment and are notto be construed as limiting the invention.

FIG. 1 shows a turbomolecular pump in longitudinal cross section, withseveral temperature sensors,

FIG. 2 shows a block diagram of the control of the turbomolecular pumpof FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 1, a turbomolecular pump 10 is illustrated that comprises a pumphousing 12 the one longitudinal end of which forms the suction side 14and the other end of which forms the delivery side and comprises a gasoutlet 16. In the pump housing 12, a pump stator 18 is arranged tointeract with a pump rotor 20. The pump rotor 20 comprises a rotor shaft22 rotatably supported in the pump housing 12 with two radial magneticbearings 24, 26 and a non-illustrated axial bearing. The rotor shaft 22and the pump rotor 20 connected therewith are driven by an electricmotor 28. The electric motor 28 and the two radial magnetic bearings 24,26 are accommodated in a common bearingmotor housing 30. The pumphousing 12 is cooled by a coolant flowing through a cooling channel 13in the pump housing 12. The turbomolecular pump 10 serves to generate ahigh-vacuum and rotates at rotational speeds up to 100,000 rpm.

On the stator side, i.e., on the side of the stationary parts, theturbomolecular pump 10 comprises several temperature sensors 32–38. Afirst temperature sensor 32 is arranged in the region of the base flangeof the pump housing 12. A second temperature sensor 34 is arranged at orin the pump stator 18. A third temperature sensor 36 is arranged at themotor 28 and measures the temperature prevailing in the region of themotor windings and the magnetic guiding plates of the motor. A fourthtemperature sensor 38 is arranged at the bearing-motor housing 30. Afurther temperature sensor may be arranged in the course of the coolingchannel 13.

The heat transferred to the pump rotor 20 by the gas heating of thecompressed gas and induced in the pump rotor 20 by the active magneticbearings 26 and the electric motor 28 is substantially dissipated fromthe pump rotor 20 to the stator-side parts by heat radiation. Apart fromtheir self-heating, the stator-side parts, i.e., the pump housing 12,the pump stator 18, the bearing-motor housing 30 as well as the magneticbearings 24,26 and the electric motor 28, are hence also heated by theheat radiated onto them by the pump rotor 20. The measurement of thetemperature and the temperature course of the mentioned stator-sideparts therefore allows conclusions with respect to the rotortemperature.

The relation between the actual temperature of the pump rotor 20 and thetemperatures of the stator-side parts measured by the temperaturesensors 32–38 can be detected by a simple experimental set-up. To thisend, a rotor temperature sensor 40 is suitably arranged on the suctionside as close to the pump rotor 20 as possible. Thus, the rotortemperature can be measured directly in the experiment so that theconnection between the rotor temperature and the temperatures measuredby the stator-side temperature sensors 32–38 can be recorded underdifferent process conditions. From the temperatures and temperaturecourses recorded by all temperature sensors 32–40, a polynomial for themotor output power P in dependence on the rotor temperature and thestator-side temperatures can be detected:P=α ₀+α₁ T ₁ ^(β) ₁+α₂ T ₂ ^(β) ₂+α₃ T ₃ ^(β) ₃ . . . α_(n) T _(n) ^(β)_(n).P is the instantaneous motor output power, T₁ to T_(n) are therespectively measured temperatures of the stator-side temperaturesensors 32–38 and the rotor temperature sensor 40. The coefficients α₀to α_(n) as well as β₁ to β_(n) are constants detected by evaluating theexperimentally measured pump rotor and pump stator temperatures. If themaximally permissible rotor temperature is put into this polynomialinstead of the measured rotor temperature, the permissible maximum motoroutput power P_(max) is detected with this polynomial.

Thus, a polynomial is presented with which the permissible maximum motoroutput power P_(max) can be calculated for a set of simultaneouslymeasured stator temperatures T₁ to T_(n), respectively.

In FIG. 2, the control of the pump rotor motor 28 is schematicallyillustrated. A control device 42 controls a motor driver 44 which, inturn, drives the windings of the electric motor 28. Via an actuator 46,a motor output power nominal value is put out to the control device 42.The control device 42 comprises a maximum output power detecting device50 and an output power limiter 52. In the maximum output power detectingdevice 50, the permissible maximum motor output power P_(max) isdetected, according to the formula indicated above, from the temperaturevalues supplied by the four temperature sensors 32–38. In the outputpower limiter 52, the motor output power nominal value supplied by theactuator 46 is restricted to the detected permissible maximum motoroutput power if the output power value indicated by the actuator 46 isgreater than the detected permissible maximum motor output power. Thus,the rotor temperature is restricted to a maximum temperature so that therotor is protected from destruction by overheating.

Aside from the cooling fluid temperature, the actual motor output power,the ambient temperature and other measurable variables can also be usedas further parameters for the detection of the permissible maximum motoroutput power.

By means of the described device, it is possible to draw conclusionswith respect to the present rotor temperature via several stator-sidetemperature sensors. To avoid an overheating of the pump rotor to atemperature above a maximum rotor temperature, a permissible maximummotor output power to which the motor output power is restricted isdetected from the detected rotor temperature. Thus, the permissiblemaximum motor output power is variable so that the capacity of the motorand the pump can be fully utilized and is only restricted in the case ofa danger of overheating.

The invention has been described with reference to the preferredembodiment. Obviously, modifications and alterations will occur toothers upon reading and understanding the preceding detaileddescription. It is intended that the invention be construed as includingall such modifications and alterations insofar as they come within thescope of the appended claims or the equivalents thereof.

1. A turbomolecular pump including: a stator, a pump rotor, a motor fordriving the pump rotor, a control device for controlling the motor, thecontrol device controlling the motor output power such that the motoroutput power does not exceed a permissible maximum motor output power, atemperature sensor for measuring the stator temperature arranged on thestator side, and the control device including a maximum output powerdetecting device which determines a variable permissible maximum motoroutput power in dependence on variations in the measured statortemperature.
 2. The turbomolecular pump according to claim 1, whereinseveral temperature sensors are provided at different sites of thestator, and the maximum output power detecting device determines thepermissible maximum motor output power in dependence on the measuredtemperatures of all temperature sensors.
 3. The turbomolecular pumpaccording to claim 1, wherein the maximum output power detecting devicehas a rotor temperature detecting device allocated thereto which detectsthe rotor temperature from the stator temperature measured by thetemperature sensor, and the maximum output power detecting devicedetermines the permissible maximum motor output power in dependence onthe detected rotor temperature.
 4. The turbomolecular pump according toclaim 1, wherein the maximum output power detecting device detects thepermissible maximum motor output power by means of a polynomial.
 5. Aturbomolecular pump including: a pump rotor, a motor for driving thepump rotor, a control device for controlling the motor, the controldevice controlling the motor output power such that the motor outputpower does not exceed a permissible maximum motor output power, atemperature sensor for measuring the stator temperature arranged on thestator side, and the control device including a maximum output powerdetecting device which determines a changeable permissible maximum motoroutput power in dependence on change in the measured stator temperature,the maximum output power detecting device comprising a characteristicsfield memory in which the permissible maximum motor output power foreach stator temperature is stored in a characteristics field.
 6. Theturbomolecular pump according to claim 1, wherein the temperature sensoris provided at a pump housing.
 7. The turbomolecular pump according toclaim 1, wherein the temperature sensor is provided at a pump stator. 8.The turbomolecular pump according to claim 1, wherein the temperaturesensor is provided at a stator-side part of the motor.
 9. Theturbomolecular pump according to claim 1, wherein the motor includes ahousing and the temperature sensor is provided at the motor housing. 10.The turbomolecular pump according to claim 1, further including: acooling channel defined in the stator, the temperature sensor beingarranged along the course of the cooling channel.
 11. A method forrestricting output power of a motor in a turbomolecular pump, said motordriving a pump rotor borne in a stator, with the method steps of:measuring a pump stator temperature, determining a permissible maximummotor output power in dependence on the measured pump statortemperature, permissible maximum motor output power varying withvariations in the measured pump stator temperature, controlling themotor to restrict the motor output power to the determined permissiblemaximum motor output power.
 12. The method according to claim 11,wherein detecting the permissible maximum motor output power includesthe steps of: calculating the pump rotor temperature from the measuredpump stator temperature, determining the permissible maximum motoroutput power from the calculated pump rotor temperature.